Device and method for connecting two wafers in a planar manner for grinding down and cutting up a product wafer

ABSTRACT

A device for connecting two wafers in a planar manner for grinding down and cutting up a product wafer has a vacuum chamber, a chuck for receiving a carrier wafer, a heating device for heating up the chuck and a vacuum-chamber cover with a vacuum-holding device, on which a product wafer can be arranged suspended above the carrier wafer. After the evacuation of the vacuum chamber, the active surface of the product wafer is dropped onto a double-sided adhesive film on the carrier wafer and is pressed into place by the rising pressure during air admission. The result is that the wafers are connected together.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation of copending InternationalApplication No. PCT/DE01/03683, filed Sep. 26, 2001, which designatedthe United States and was not published in English.

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION

[0002] The invention relates to a device for connecting two wafers in aplanar manner and to a method for connecting two wafers in a planarmanner for grinding down and cutting up a product wafer.

[0003] U.S. Pat. No. 6,045,073 discloses a method for grinding downsemiconductor chips, in which the active surfaces of the chips areinitially connected electrically to a contact surface of a leadframe viacontact bumps and the edge regions thereof are encapsulated in asilicone composition. Subsequently, the silicon remains are removed fromthe rear side of the chips and the rear side of a chip is subjected to aplasma etching process, in order to etch the chip down to a few μm.

[0004] This method has the disadvantage that the etching down cannot beused simultaneously for many chips on a wafer of a large surface area,but is merely restricted to relatively small surface areas of anindividual chip. The restriction of the previous solutions to theetching down of individual chips is essentially attributable to the factthat it is problematic for a carrier of a large surface area to beconnected in a planar manner to a wafer of a large surface area. Evenminor deviations in the parallelism of the carrier and the wafer resultin considerable differences in thickness from one edge region of thewafer to the other edge region, so that uniform etching down of theentire wafer to a few μm cannot be accomplished with the previouslyknown method, especially since commercially available wafers have adiameter of 150 to 300 mm.

SUMMARY OF THE INVENTION

[0005] It is accordingly an object of the invention to provide a devicefor connecting two wafers in a planar manner and a method for connectingtwo wafers in a planar manner for grinding down and cutting up a productwafer, which overcome the above-mentioned disadvantages of the prior artapparatus and methods of this general type.

[0006] In particular, it is an object of the invention to provide adevice and a method for connecting two wafers in a planar manner suchthat a product wafer can be ground down and then cut up without havingto first cut up the product wafer into chips and then etch the rearsides of the chips while each chip is in a corresponding carriermaterial or in a corresponding semiconductor package. In addition, theinvention is intended to provide a method with which it is possible fortwo wafers to be connected in a planar manner with high precision,without instances of warpage, bowing, sloping, or other large-areadefects occurring during the connection.

[0007] With the foregoing and other objects in view there is provided,in accordance with the invention, a device for connecting two wafers oneon top of another in a planar manner. The device includes: a vacuumchamber having a chamber volume and an evacuating device for the chambervolume; a chuck having an evacuating device for receiving a carrierwafer having a surface with a double-sided adhesive film or an adhesivelayer; a heating device for heating up the chuck; and a vacuum-chambercover having a vacuum-holding device for holding a product wafer havinga surface. The vacuum-holding device is configured on the vacuum-chambercover for suspending the product wafer in the vacuum chamber at adistance above the carrier wafer such that the surface of the productwafer and the surface of the carrier wafer are congruent before beingconnected.

[0008] With the foregoing and other objects in view there is alsoprovided, in accordance with the invention, a method for connecting aproduct wafer and a carrier wafer in a planar manner. The methodincludes steps of: attaching a double-sided adhesive film or an adhesivelayer onto the carrier wafer; placing the carrier wafer betweenpositioning pins on a chuck of a vacuum chamber; placing the productwafer between guiding pins of a vacuum-holding device of avacuum-chamber cover and fixing the product wafer by opening a firstvacuum valve connecting the vacuum-holding device to an evacuatingdevice; closing the vacuum chamber and evacuating the vacuum chamber toan equal or higher vacuum than a vacuum of the vacuum-holding device byopening a second vacuum valve; and after closing at least the firstvacuum valve and the second vacuum valve and opening air-admittingorifices, admitting air into the vacuum chamber such that the carrierwafer and the product wafer are simultaneously pressed onto each otherin a planar manner by rising pressure in the vacuum chamber.

[0009] According to the invention, the device for connecting two wafersone on top of the other in a planar manner includes a vacuum chamberwith an evacuating device for the chamber volume, a chuck with anevacuating device for receiving a carrier wafer with double-sidedadhesive film or an adhesive layer provided on one side of the carrierwafer, a heating device for heating up the chuck, and a vacuum-chambercover with a vacuum-holding device for a product wafer. Thevacuum-holding device is arranged on the vacuum-chamber cover in such away that the product wafer is arranged suspended in the vacuum chamberat a distance above the carrier wafer, with congruent surface areas,before the product wafer and the carrier wafer are connected.

[0010] This device has the advantage that the distance or intermediatespace between a product wafer held by the vacuum-holding device and theadhering surface of the double-sided adhesive film on the carrier wafer,arranged underneath the product wafer on a chuck and having adouble-sided adhesive film, can be evacuated completely, so that nocontamination and no microscopic gas bubbles can impair the planaritybetween the two wafers. A further advantage of this device is that theproduct wafer held by a vacuum-holding device is arranged in a suspendedmanner, and consequently with its active surface directed downward, sothat no dust or other contamination particles can settle on the surfacebefore the surface comes into contact with the double-sided adhesivefilm. A further advantage of this invention is that, at the instant atwhich extraction pumping causes the vacuum in the vacuum chamber tobecome less than the vacuum by which the product wafer is held suspendedover the carrier wafer, the product wafer automatically falls down fromits vacuum securement and can be set down flat on the double-sidedadhesive film without any gas pocket. A further advantage of this deviceis that the product wafer can overcome the distance between the productwafer and the double-sided adhesive film in free fall without anycanting and falls onto the film in a planar manner.

[0011] To ensure reliable setting down of the product wafer on thedouble-sided adhesive film of the carrier wafer, in one embodiment ofthe invention, the vacuum-holding device has guiding pins, whichprotrude perpendicularly from its surface and are conically shaped.These cone bases area of the guiding pins are positioned on thevacuum-holding device, while the cone tips protrude from thevacuum-holding device. This arrangement has the advantage that theproduct wafer can fall down unhindered, and yet guided, by the guidingpins from the vacuum-holding device on the vacuum-chamber coverautomatically and in a plane-parallel manner, without canting orcatching on the guiding pins, since in this embodiment they areconically shaped.

[0012] To ensure this reliable guidance, the guiding pins of thevacuum-holding device are arranged in the edge region of the productwafer. At the same time, the guiding pins of the vacuum-holding deviceare arranged with respect to the chuck which carries the carrier waferin such a way that precise and aligned setting-down and receiving of theproduct wafer on the double-sided adhesive film or on the adhesive layerof the carrier wafer on the chuck is ensured. This is achieved on theone hand by the vacuum-chamber cover being precisely brought togetherwith the vacuum chamber and on the other hand by the product wafer beingplaced exactly between the guiding pins.

[0013] For controlling the individual method steps, vacuum valves areprovided in the vacuum lines and in the connections between one or moreevacuating devices.

[0014] In a further embodiment of the invention, the device includes afirst vacuum valve configured between an evacuating device of thevacuum-holding device and the vacuum-chamber cover, a second vacuumvalve configured between the evacuating device for the chamber volumeand the vacuum chamber, and a third vacuum valve configured between theevacuating device of the chuck and the chuck. With these vacuum valves,the individual process steps can be controlled and the positions both ofthe carrier wafer and of the product wafer can be reliably handled.

[0015] In one embodiment of the invention, at least three guiding pinsare arranged on the vacuum-holding device, since three guiding pinsallow clear fixing to take place in the X and Y directions. An improvedversion of the device provides that at least five guiding pins arearranged on the vacuum-holding device. With five guiding pins for onewafer in each case, the latter is reliably secured against slipping,canting, shifting or becoming displaced in some other way.

[0016] A further embodiment of the invention provides that the guidingpins are of a length which corresponds at least to the thickness of theproduct wafer plus the distance between the product wafer and thedouble-sided adhesive film or the adhesive layer, and which is less thanthe distance between the surface of the vacuum-holding device of thevacuum cover and the surface of the chuck. In this case, the distancebetween the product wafer suspended from the vacuum-holding device andthe carrier wafer lying on the chuck is dimensioned in such a way thatreliable vacuum-drying of the opposing surfaces is possible, so that thesurface of the double-sided adhesive film can completely outgas, and theopposite side of the product wafer has a completely dry surface and,after the evacuation of the vacuum chamber, there are no gases betweenthe surfaces to be adhesively bonded. The distance depends, furthermore,on the size of the opposing surface areas. The larger these surfaceareas are, the greater the extraction-pumping cross section must bechosen, and consequently the greater the distance between the wafersmust be set. In a further embodiment of the invention, the distance for6 to 12-inch wafers (150 to 300 mm) lies between 3 and 15 millimeters.In the case of wafers of up to 6 inches, the distance can be reduced toone millimeter.

[0017] A further embodiment of the invention provides that the cone baseareas of the guiding pins have a diameter of 200 μm to 1200 μm and atthe cone tips have a diameter of between 100 and 500 μm. Such slenderand thin pins have the advantage that they are extremely compliant andinduce the lowest possible stresses in the product wafer during theguidance of the product wafer.

[0018] In a further embodiment of the invention, the vacuum-holdingdevice has depressions that can be connected to an evacuating device viaa first vacuum valve. Depressions of this type are formed in thevacuum-holding device as concentric grooves and have, in their groovebase, drilled holes that communicate with the evacuating device for thevacuum-holding device. With this embodiment it is ensured that the rearside of the product wafer is subjected to a vacuum over a large surfacearea and is suspended from the vacuum-chamber cover in a planar manneron the surface of the vacuum-holding device.

[0019] A further embodiment of the invention provides that the chuck haspositioning pins, which protrude perpendicularly from the surface of thechuck. These positioning pins may be designed in a way similar to theguiding pins for the product wafer, whereby the same advantages are alsoobtained for the carrier wafer. On the other hand, the carrier waferdoes not have to be guided over the distance between the carrier waferand the product wafer. To this extent, the positioning pins may alsohave cylindrical shapes and be of a length that is less than or equal tothe thickness of the carrier wafer. Conical forming of the positioningpins allows them to protrude beyond the carrier wafer and contribute tothe guiding into place of the product wafer. For this purpose, thepositioning pins of the chuck are arranged with respect to thevacuum-holding device in such a way that precise and alignedsetting-down and receiving of the product wafer on the double-sidedadhesive film or on the adhesive layer of the carrier wafer on the chuckis ensured.

[0020] In particular if both guiding pins and positioning pins protrudebeyond the respective wafer surfaces, they are not only conically shapedin an advantageous way, but should not exceed the thickness of therespective wafer plus the distance lying between the wafers, to ensurethat neither the guiding pins nor the positioning pins touch thesurfaces of the chuck lying opposite or the vacuum-holding device lyingopposite.

[0021] In a preferred embodiment, the positioning pins are cylindricallyformed and are of a smaller length than the thickness of the carrierwafer. This is permissible, since in this embodiment they do notcontribute to the guiding of the product wafer, but align the carrierwafer in relation to the product wafer exactly and with congruentsurfaces.

[0022] If the sum of the lengths of the positioning pins and guidingpins together exceeds the distance between the surface of the chuck andthe surface of the vacuum-holding device, in a further embodiment of theinvention, the positioning pins are arranged offset from the guidingpins with respect to the edge of the wafers to be connected. It isconsequently ensured at the same time that positioning and guiding pinscannot collide with one another. Both the chuck and the vacuum-holdingdevice have fastening possibilities for the positioning pins and guidingpins, which allow adaptation to the respective size of a wafer.

[0023] In a further embodiment of the invention, the device has aheater, which is arranged on the chuck and permits heating of the chuckto between 60 and 200° C. The vacuum-holding device may also have afurther heater, to assist outgassing of the surface of the productwafer.

[0024] A method for connecting two wafers in a planar manner forgrinding down and cutting up a product wafer, the one wafer being acarrier wafer with a double-sided adhesive film or with an adhesivelayer and the second wafer being a product wafer, has the followingmethod steps:

[0025] drawing the double-sided adhesive film or the adhesive layer ontothe carrier wafer;

[0026] placing the carrier wafer between positioning pins on a chuck ofa vacuum chamber;

[0027] placing the product wafer between guiding pins of avacuum-holding device of a vacuum-chamber cover and fixing the productwafer by opening a first vacuum valve that connects the vacuum-holdingdevice to an evacuating device;

[0028] closing the vacuum chamber and evacuating the vacuum chamber toan equal or higher vacuum than the vacuum of the vacuum-holding deviceby opening a second vacuum valve;

[0029] admitting air into the vacuum chamber after closing the vacuumvalves and opening air-admitting orifices, with the waferssimultaneously being pressed one onto the other in a planar manner bythe rising pressure in the vacuum chamber.

[0030] This method has the advantage that, without mechanical aids,after closing the vacuum chamber and evacuating the vacuum chamber tothe same or a higher vacuum than the vacuum of the vacuum-holdingdevice, the product wafer, held in a suspended manner by thevacuum-holding device on the vacuum-chamber cover, falls down from thevacuum-holding device and drops in a planar manner onto the surfacefacing it of the double-sided adhesive film on the carrier wafer. Duringthis falling, the product wafer is guided by the guiding pins arrangedaround its edge, so that it is neither laterally displaced nor canted asit falls down. For this purpose, the guiding pins, as stated above, areconically shaped and, on account of their length, bridge the distancebetween the vacuum-holding device and the surface of the double-sidedadhesive film on the carrier wafer. Furthermore, this method has theadvantage that the active surface of the product wafer can be held at adistance from the surface of the double-sided adhesive film on thecarrier wafer, which for its part is arranged on the chuck, so thatduring the evacuation phase of the vacuum chamber the intermediate spacebetween the two surfaces can outgas completely and the surfaces of theproduct wafer and the double-sided adhesive film to be brought togetherare vacuum-dried.

[0031] The pumping cross section between the two surfaces for theapplied and increasing vacuum of the vacuum chamber can be adapted bythe distance between the two surfaces to the requirements of theoutgassing and vacuum drying and to the requirement for completeevacuation of the intermediate space between the product wafer and thedouble-sided adhesive film. The extraction-pumping cross section is inthis case the generated surface of the intermediate space between thesurface of the product wafer and the surface of the double-sidedadhesive film along the outer edge of the product wafer. Consequently,the extraction-pumping cross section is determined by the distancebetween the product wafer surface and the double-sided adhesive film andalso the size of the product wafer. A further advantage of the inventivedevice is that the extraction-pumping cross section can be adapted tothe requirements of the process by increasing the distance between theproduct wafer and the double-sided adhesive film. For instance, toreduce the extraction-pumping time and consequently the production time,it is possible to increase the extraction-pumping cross section byincreasing the distance when there is adequate capacity of theevacuating device and reduce the extraction-pumping cross section whenthe extraction-pumping time lasts longer on account of a reducedcapacity of the evacuating device.

[0032] In an example of how the method is carried out, the step ofgrinding down the product wafer, which has been adhesively attached in aplanar manner, to a thickness below 100 μm is additionally carried out.On account of the method of connecting a carrier wafer to the productwafer to be ground down, with thickness variations limited to a few μmover the size of the product wafer surface area, this grinding down canbe performed in a corresponding automatic grinding-down machine. Everydeviation during the connecting of the two wafers in a planar mannerfrom their plane-parallelism has an effect on the uniformity of thethickness of the ground-down wafer. Since, however, on account of themethod, the active surface area of the product wafer is brought onto thedouble-sided adhesive film under a vacuum, there is no possibility ofcausing gas bubble formations between the wafers by residual gaspockets. These gas bubble formations if present could be the cause of anon-planar connection between the product wafer and the carrier wafer.

[0033] In a further example of how the invention is carried out, theproduct wafer that has been ground-down to below 100 μm is etched downto a thickness of as little as 15 μm. This method variant has theadvantage that, with wafers becoming increasingly thinner,etching/mechanical grinding down is no longer carried out much below 100μm and instead there is a trend toward thinning down purely by etching,without any mechanical loading, for thinning to 15 μm. After etchingdown the product wafer, it is still connected, as before, to the carrierwafer, so that it is mechanically supported by the carrier wafer.

[0034] Cutting up the product wafer into individual chips can beperformed both with and without the adhesively attached carrier wafer.If the carrier wafer is separated from the product wafer before thecutting or sawing of the product wafer into individual chips, theproduct wafer connected to the carrier wafer is still adhesively fixedbeforehand onto a saw frame covered with film, and then the carrierwafer is heated by heating up a chuck above the release temperature forthe film or the adhesive, for example, to at least 120° C., fordetaching the double-sided adhesive film and for removing the carrierwafer. After that, the ground-down and etched product wafer in thecovered saw frame can be cut up into individual chips. The releasetemperature is understood as meaning a temperature at which theadhesiveness subsides and the product wafer can be detached from thecarrier wafer. The release temperature may well be below the meltingtemperature of the double-sided adhesive film or the adhesive layer.

[0035] In the case of a variant of the method for cutting up theground-down product wafer into individual chips, initially the waferassembly including a ground-down and etched-down product wafer and acarrier wafer undergoes a cutting-up step, in which the ground-down andetched-down product wafer is cut up into chips, and subsequently theentire composite wafer is adhesively fixed onto a carrier film. Theground-down and etched-down, and now separated, chips are adhesivelyfixed onto the carrier film. To remove the carrier wafer holding thechips, the assembly including the composite wafer and the double-sidedadhesive film is heated up to the melting temperature of thedouble-sided adhesive film and the carrier wafer is pulled off from thecomplete assembly, so that subsequently the ground-down and etched-downchips stuck on a carrier film are available for further processing. Thismethod has the advantage that the cutting up of the ground-down andetched-down product wafer into chips can be carried out by sawingcutting-up methods, in which a wafer is divided into chips.

[0036] A further variant provides that, even before it is introducedinto the device for connecting it to a carrier wafer, the surface of theproduct wafer is provided with sawing grooves that already divide up thesurface of the product wafer into individual chip areas to a depth ofdown to 100 μm, so that after the grinding-down and etching-down of theproduct wafer the latter is automatically in a form in which it is cutup into individual chips on the carrier wafer.

[0037] Consequently, the method has the advantage that all threevariants of cutting up a product wafer into chips involving grindingdown and etching down the product wafer can be carried out. The methodfor connecting a product wafer to a carrier wafer in a planar manner bya double-sided adhesive film consequently improves the prospects ofsuccessfully grinding-down, etching-down, and cutting up a product waferinto individual ground-down and etched-down chips.

[0038] Consequently, with the method, the product wafer and the carrierwafer are bonded by a double-sided adhesive film in a vacuum chamber. Aground dummy wafer may be used as the carrier wafer. The double-sidedadhesive film is used as the connecting adhesive. First, a carrier waferwith the adhesive which will later connect the device wafer or productwafer and the carrier wafer is introduced into the chamber.

[0039] To center the product wafer and the carrier wafer without anyoffset, conical pins are used. The device wafer is sucked into place onthe wafer rear side by a vacuum (vacuum 1). The chamber is thenevacuated (vacuum 2). By equalizing the pressure, vacuum 1 loses itsholding force and the product wafer falls onto the carrier, guided bythe conically tapering pins or guiding pins. By subsequently admittingair, the product wafer is then uniformly loaded and pressed onto thecarrier wafer, which leads to a firm connection. No punch is used forpressing the device wafer.

[0040] Thinning the product wafers far below 100 micrometers requires acarrier wafer that is firmly connected to the product wafer during thethinning, and gives it the necessary stability. Dummy wafers or ceramicwafers may be used as materials for the carrier wafer. It is leastexpensive to use a pre-ground dummy wafer as the carrier wafer. Thepre-grinding ensures a constant thickness, uniformity, and surfacequality of the carrier wafer.

[0041] The product wafer and the carrier wafer are adhesively attachedone on top of the other by a double-sided adhesive, thermally releasablefilm. After the thinning of the carrier wafer, the product wafer isreleased again by heat exposure. At about 120 degrees Celsius, thedouble-sided adhesive film loses its adhesive force. This film can bestored in the rolled-up state with two covering films.

[0042] As the first product wafer, finger-tip wafers were thinned to 80micrometers, 60 micrometers and 40 micrometers. The wafers were sawninto beforehand (bevel cut before thinning). In this case, the separatedchips were subsequently supplied on leadframes.

[0043] The basic module of the device is a vacuum chamber, which isequipped for vacuum-connecting two wafers. The device for connectingwafers in a planar manner permits carrier wafers on product wafers to behandled with a throughput of about 15 wafers per hour. The compositewafers produced by the device can still be handled when the productwafer is in an extremely thin state. The product wafer on the compositewafer can be ground down to about 70 micrometers. Further removal of theproduct wafer can be performed by etching.

[0044] Other features which are considered as characteristic for theinvention are set forth in the appended claims.

[0045] Although the invention is illustrated and described herein asembodied in a device and method for connecting two wafers in a planarmanner for grinding down and cutting up a product wafer, it isnevertheless not intended to be limited to the details shown, sincevarious modifications and structural changes may be made therein withoutdeparting from the spirit of the invention and within the scope andrange of equivalents of the claims.

[0046] The construction and method of operation of the invention,however, together with additional objects and advantages thereof will bebest understood from the following description of specific embodimentswhen read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0047]FIG. 1 is a basic diagram of a device for connecting two wafersone on top of the other in a planar manner;

[0048]FIG. 2 is a portion of a device for connecting two wafers one ontop of the other in a planar manner; and

[0049]FIG. 3 is a flow diagram showing the method steps of an example ofhow to carry out the method for connecting two wafers in a planar mannerfor grinding down and cutting up a product wafer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0050] Referring now to the figures of the drawing in detail and first,particularly, to FIG. 1 thereof, there is shown a basic diagram of adevice for connecting two wafers 1, 2 one on top of the other in aplanar manner. In FIG. 1, the reference numeral 3 designates a vacuumchamber, which is connected to an evacuating device 104 for the chambervolume. The reference numeral 4 designates a chuck, which is likewiseconnected to an evacuating device 102 and can receive a carrier wafer 2on its surface 12. The carrier wafer 2 has an edge region 14. Thereference numeral 5 shows a double-sided adhesive film, which in FIG. 1is connected by one of its adhesive surfaces to the carrier wafer 1.Numeral 8 designates a heating device, which is capable of heating upthe chuck 4 to the melting temperature of the double-sided adhesive film5.

[0051] The vacuum chamber 3 is closed off in the downward direction by avacuum base plate 29 having a number of lead-throughs. The lead-throughs30 and 31 are power lead-throughs for the heating device 8 of the chuck4. Arranged in the center of the vacuum base plate 29 is alifting-turning lead-through 32 that can be used to adjust the height ofthe chuck 4 and, if required, to turn the chuck 4. This lifting-turningdevice has a piece of pipe 33 which leads from a vacuum valve 22 to theevacuating device 102 of the chuck 4. The pipe 33 also serves as avacuum line 34. Air can be admitted to the interior of the piece of pipe33 via an air-admission orifice 28, when the vacuum valve 22 is closed,and consequently the connection to the evacuating device 102 isinterrupted.

[0052] The vacuum base plate 29 additionally has a pipe connector 35,via which the vacuum chamber 3 can be connected to the evacuating device104 via a vacuum line 45 after the vacuum valve 7 has been opened. Aircan be admitted to the vacuum chamber 3 via an air-admission orifice 27when the vacuum valve 7 is closed. Arranged on the vacuum base plate 29is a piece of pipe 37 with a lower flange 38 and an upper flange 39,which forms a vacuum chamber wall 35. The lower flange 38 is connectedin a vacuum-tight manner to the vacuum base plate 29 by means of anO-ring 40. The upper flange 39 carries a vacuum-chamber cover 18, whichfor its part is connected in a vacuum-tight manner to the upper flange39 by means of an 0ring 41.

[0053] Arranged on the vacuum-chamber cover 18 is a vacuum-holdingdevice 19 having a surface 20 on which the rear side of a product wafercan be held suspended so that its active surface 42 is at a distance afrom the second adhesive surface of the double-sided adhesive film 5.The vacuum-holding device 19 is connected to an evacuating device 106via a vacuum line 44, a pipe connector 43, and a vacuum valve 6, andthese components are used to evacuate the vacuum-holding device 19. Aircan be admitted to the vacuum-holding device 19 via an air-admissionorifice 26 when the vacuum valve 6 is closed.

[0054] Arranging a vacuum-holding device 19 on a vacuum-chamber cover 18makes it possible to arrange a product wafer 1 with its active surface42 lying opposite a carrier wafer at a distance a, so that theintermediate space formed by the distance a between the product wafer 1and the carrier wafer 2 can be completely evacuated and outgassed and,if required, the surfaces of the product wafer to be connected to one ofthe surfaces of the double-sided adhesive film 5 can be vacuum-driedbefore connecting. It is consequently possible to bring these surfacesonto each other only after this vacuum preparation of the surfaces, inthat for example extraction pumping via the vacuum valve 7 and thevacuum line 45 causes the vacuum in the vacuum chamber 3 to becomegreater than the vacuum which is acting on the vacuum-holding device 19via the vacuum line 44 and the vacuum valve 6. This is because, with anequal or greater vacuum in the vacuum chamber in comparison with thevacuum of the vacuum-holding device 19, the product wafer falls from itssuspended position with its active surface onto the double-sidedadhesive film 5 and, during the subsequent admission of air to thevacuum chamber via the air-admission orifice 27 with the vacuum valves5, 7 and 22 closed at the same time, is pressed by the rising pressurein the vacuum chamber 3 onto the carrier wafer.

[0055]FIG. 2 shows a detail A of a device for connecting two wafers 1and 2 one on top of the other in a planar manner. For this purpose, thecarrier wafer 2 is arranged with its thickness D on the chuck 4 shown inFIG. 1, which is shown here in the form of a detail and partly in crosssection. The chuck 4 can be heated up by a heating device 8.Furthermore, the chuck 4 can be evacuated in the direction of the arrowB, so that the carrier wafer 2 can be held on the chuck 4 using thedrilled vacuum holes 17. The exact position of the carrier wafer 2 onthe chuck 4 is determined by means of positioning pins 23, 24, of whichone positioning pin 24 is shown in this detail A. A positioning pin 24of this type may have a cylindrical shape, as long as its length doesnot exceed the thickness D of the carrier wafer 2 plus the thickness hof the double-sided adhesive film 5. In the embodiment depicted in thedetail A, the positioning pin 24 has a conical shape and is located withits cone base area 25 on the surface 12 of the chuck 4 and protrudeswith its cone tip 47 from the surface 12. The conical configuration ofthe positioning pin 24 has the advantage that the cone tip 47 cancontribute to the guiding and positioning of the product wafer 1.

[0056] The detail A additionally shows a partial cross section of thevacuum-holding device 19, with which a product wafer 1 can be held withits rear side 48 on the vacuum-chamber cover 18, which is shown inFIG. 1. The vacuum-holding device 19 is evacuated in the direction ofthe arrow C, whereby the rear side 48 of the product wafer 1 is pressedonto the surface 20 of the vacuum-holding device. For this purpose, thevacuum-holding device 19 has drilled vacuum holes 17 (also identifiedwith numerals 15 and 16 in FIG. 1), which, in an embodiment not shown,can be made in the vacuum-holding device 19 in concentrically arrangedgrooves. Exact positioning of the product wafer 1 during the holding bythe vacuum-holding device 19 and during the method of connecting twowafers is achieved by conical guiding pins 9 and 10, of which theconical guiding pin 10 is shown in the detail A. Its length bridges thedistance a between the surface 42 of the product wafer 1 and the surfaceof the double-sided adhesive film 5. The conical form of the guiding pin10 ensures that, when the product wafer falls down onto the double-sidedadhesive film 5, canting of the wafer on the guiding pin 10 is avoided.Furthermore, the detail A shows that the guiding pin 10 of the holdingdevice 20 is arranged offset from the positioning pin 24 on thecircumference of the wafer, so that the pins are not a hindrance duringthe adhesive attachment and connecting of the product wafer 1. Theguiding pin 10 has a cone tip 13.

[0057] Since the device which is shown in FIG. 1 has the possibility ofadjusting the height of the chuck 4 via the lifting and turninglead-through 32, it may well be of advantage to align the guiding pinsand the positioning pins exactly with one another, so that it is ensuredthat, when the chuck 4 is moved up in the direction of thevacuum-holding device 19, a minimum distance a is ensured and the twowafers 1, 2 are not inadvertently pressed onto each other before theevacuation.

[0058] A further advantage of the device shown in FIGS. 1 and 2 is thatthe distance a can be varied during the operation of connecting twowafers 1, 2 for grinding down and subsequently cutting up a productwafer 1. For instance, the extraction-pumping cross section can be keptlarge at the beginning of the operation, in that the lifting device ofthe chuck 4 is arranged in its lowest position, and the distance a canbe reduced to a few millimeters, by raising the chuck using thelifting-turning device 32, before the falling down of the product wafer1, i.e. as long as the vacuum in the vacuum chamber 3 has not yetreached the vacuum of the vacuum-holding device 19. By bringing the twosurfaces to be connected together, that is the active surface 48 of theproduct wafer 1 and the free surface of the double-sided adhesive film5, the risk that the falling-down product wafer 1 will undergo cantingis minimized when there is an equal or higher vacuum in the vacuumchamber 3 in comparison with the vacuum of the vacuum-holding device 19.

[0059]FIG. 3 is a flow diagram showing the steps of an example of amethod for connecting two wafers in a planar manner for grinding downand cutting up a product wafer 1. In a first method step 50, the firstcovering film of the two covering films of a double-sided adhesive film5 is removed. In the next method step 51, the exposed surface of thedouble-sided adhesive film 5 can be drawn onto the carrier wafer 2. Thisdrawing of a double-sided adhesive film onto a carrier wafer may alreadybe carried out fully automatically under a vacuum. The drawing of thedouble-sided adhesive film 5 onto the carrier wafer 2 is followed bymethod step 52, in which the carrier wafer 2 with the double-sidedadhesive film is placed between the pins or positioning pins 23, 24 ontoa chuck 4 of a semi-automatic machine, as shown in FIG. 1.

[0060] In the next step 53, the second covering film, which is stilllocated on the double-sided adhesive film, can be removed from thelatter. For this purpose, the carrier wafer 2 may already be fixed onthe chuck 4 by evacuating the chuck 4. In step 54, the product wafer 1to be ground is then sucked into place between the pins or guiding pinsof the vacuum-chamber cover 18 of the semi-automatic machine, as shownin FIG. 1. After closing and evacuating the vacuum chamber 3 in step 55,the product wafer 1, which was held suspended from the vacuum-holdingdevice 19, falls onto the adhesive surface of the double-sided adhesivefilm 5. By admitting air to the vacuum chamber 3, in method step 56, theproduct wafer 1 is deposited with its active surface 42 on the carrierwafer 2 via the double-sided adhesive film 5.

[0061] After removing the composite wafer produced in this way,including the product wafer 1 and the carrier wafer 2 with thedouble-sided adhesive film 5 lying in between, there may follow furthermethod steps, for example, grinding down the product wafer, separatingthe product wafer and the carrier wafer, and cutting up the productwafer into chips. For this purpose, in method step 57 the wafer isground down to <100 μm and in method step 58 the product wafer 1 isetched to a minimum of 40 μm. This minimum of 40 μm does not constitutea limit, but is reached in this example of how the method is carriedout. Etching down can be carried out over a large surface area, evendown to thicknesses of 15 μm and below. Subsequently, in method step 59,the wafer assembly including the product wafer 1 and the carrier wafer 2is adhesively attached with the ground-down wafer 1 on a saw framecovered with film, with the double-sided adhesive film 5 lying inbetween. After that, separating the product wafer 1 and the carrierwafer 2 is performed using a heatable chuck 4, at 120° C. for example,in method step 60, and finally the ground-down product wafer 1 is sawninto chips in the saw frame covered with a film.

[0062] Apart from this example of how to carry out a method forconnecting two wafers in a planar manner for grinding down and cuttingup a product wafer 1 into chips, there are further variants, which havealready been described above. In particular, with the device shown inFIG. 1, the pumping cross section can be varied during the closing andevacuating of the vacuum chamber in step 55 by using a liftinglead-through 32 in the vacuum base plate 29 of the device shown in FIG.1 to make the chuck 4 be kept initially in a position away from thevacuum-holding device 19 and brought into position so that the distancea only has a few millimeters between the product wafer 1 and the carrierwafer 2 only shortly before the falling down of the product wafer 1.

[0063] The positioning of the positioning pins and of the guiding pinsat the edges of the wafers may be variable and respectively adapted tothe size and shape of the wafers to be connected. The cutting up of theproduct wafer 1 into chips may be performed before separating theproduct wafer 1 from the carrier wafer 2, so that, when the productwafer 1 is separated from the carrier wafer 2, this already has theeffect that only chips are obtained for further processing. Othervariations, obvious to a person skilled in the art, are possible withoutdeparting from the scope of invention as defined by the claims.

We claim:
 1. A device for connecting two wafers one on top of another ina planar manner, the device comprising: a vacuum chamber having achamber volume and an evacuating device for said chamber volume; a chuckhaving an evacuating device for receiving a carrier wafer having asurface with a double-sided adhesive film or an adhesive layer; aheating device for heating up said chuck; and a vacuum-chamber coverhaving a vacuum-holding device for holding a product wafer having asurface; said vacuum-holding device configured on said vacuum-chambercover for suspending the product wafer in said vacuum chamber at adistance above the carrier wafer such that the surface of the productwafer and the surface of the carrier wafer are congruent before beingconnected.
 2. The device according to claim 1, wherein: saidvacuum-holding device has a surface and a plurality of guiding pinsprotruding perpendicularly from said surface of said holding device; andsaid plurality of guiding pins being conically shaped.
 3. The deviceaccording to claim 2, wherein each of said plurality of guiding pins hasa conical base area positioned on said vacuum-holding device and a conetip protruding from said vacuum-holding device.
 4. The device accordingto claim 2, in combination with the product wafer, wherein: the productwafer has an edge region and is being held on said vacuum-holdingdevice; and said plurality of guiding pins are configured near the edgeregion of the product wafer.
 5. The device according to claim 2, whereinsaid vacuum-holding device has a plurality of guiding pins configured,in relation to said chuck, for ensuring that when the carrier wafer ison said chuck and when the product wafer is being received on thedouble-sided adhesive film or on the adhesive layer of the carrierwafer, the product wafer is received in a precise and aligned manner. 6.The device according to claim 1, further comprising: a first vacuumvalve, a second vacuum valve, and a third vacuum valve; saidvacuum-holding device having an evacuating device; said first vacuumvalve configured between said evacuating device of said vacuum-holdingdevice and said vacuum-chamber cover; said second vacuum valveconfigured between said evacuating device of said vacuum chamber andsaid vacuum chamber; and said third vacuum valve configured between saidevacuating device of said chuck and said chuck.
 7. The device accordingto claim 1, wherein said vacuum-holding device includes at least threeguiding pins.
 8. The device according to claim 1, wherein saidvacuum-holding device includes at least five guiding pins.
 9. The deviceaccording to claim 1, in combination with the product wafer and thecarrier wafer, wherein: the product wafer has a thickness; saidvacuum-holding device has a surface; said chuck has a surface; saidvacuum-holding device includes a plurality of guiding pins that have alength of at least the thickness of the product wafer plus a distancebetween the product wafer and the double-sided adhesive film or theadhesive layer of the carrier wafer on said chuck; and said length ofeach of said plurality of said guiding pins of said vacuum-holdingdevice is less than a distance between said surface of saidvacuum-holding device of said vacuum-chamber cover and said surface ofsaid chuck.
 10. The device according to claim 1, wherein: saidvacuum-holding device includes a plurality of guiding pins; and each ofsaid plurality of said guiding pins of said vacuum-holding device has aconical base area with a diameter of 200 to 1200 micrometers.
 11. Thedevice according to claim 1, wherein: said vacuum-holding deviceincludes a plurality of guiding pins; and each of said plurality of saidguiding pins of said vacuum-holding device has a cone tip with adiameter of 100 to 500 micrometers.
 12. The device according to claim 1,wherein: said vacuum-holding device has a first vacuum valve and aplurality of depressions; and said vacuum-holding device has anevacuating device; and said first vacuum valve is for connecting saidplurality of depressions of said vacuum-holding device to saidevacuating device of said vacuum-holding device.
 13. The deviceaccording to claim 12, wherein: said plurality of depressions areconcentric grooves formed in said vacuum-holding device; and each ofsaid plurality of depressions has a groove base with a drilled holecommunicating with said evacuating device of said vacuum-holding device.14. The device according to claim 1, wherein: said chuck has a surface;and said chuck includes a plurality of positioning pins protrudingperpendicularly from said surface of said chuck.
 15. The deviceaccording to claim 14, wherein: said plurality of positioning pins areconically shaped; and each of said plurality of positioning pins has acone base area positioned on said surface of said chuck and a cone tipprotruding from said surface of said chuck.
 16. The device according toclaim 14, in combination with the carrier wafer, wherein: the carrierwafer has an edge region and is being held on said chuck; and saidplurality of positioning pins of said chuck are configured near the edgeregion of the carrier wafer.
 17. The device according to claim 14,wherein said positioning pins of said chuck are configured, in relationto said vacuum-holding device, for ensuring that when the carrier waferis on said chuck and when the product wafer is being received on thedouble-sided adhesive film or on the adhesive layer of the carrierwafer, the product wafer is received in a precise and aligned manner.18. The device according to claim 14, in combination with the carrierwafer, wherein: the carrier wafer has a thickness; and each of saidplurality of positioning pins of said chuck has a length that is lessthan or equal to the thickness of the carrier wafer.
 19. The deviceaccording to claim 14, in combination with the carrier wafer and theproduct wafer, wherein: the carrier wafer has an edge and is being heldon said chuck; the product wafer has an edge and is being held by saidvacuum holding device; the carrier wafer and the product wafer will beconnected together; said vacuum-holding device includes a plurality ofguiding pins; and said plurality of positioning pins are configuredoffset from said plurality of guiding pins with respect to the edge ofthe carrier wafer and the edge of the product wafer.
 20. A method forconnecting a product wafer and a carrier wafer in a planar manner, whichcomprises: attaching a double-sided adhesive film or an adhesive layeronto the carrier wafer; placing the carrier wafer between positioningpins on a chuck of a vacuum chamber; placing the product wafer betweenguiding pins of a vacuum-holding device of a vacuum-chamber cover andfixing the product wafer by opening a first vacuum valve connecting thevacuum-holding device to an evacuating device; closing the vacuumchamber and evacuating the vacuum chamber to an equal or higher vacuumthan a vacuum of the vacuum-holding device by opening a second vacuumvalve; and after closing at least the first vacuum valve and the secondvacuum valve and opening air-admitting orifices, admitting air into thevacuum chamber such that the carrier wafer and the product wafer aresimultaneously pressed onto each other in a planar manner by risingpressure in the vacuum chamber.
 21. The method according to claim 20,which further comprises, after the carrier wafer and the product waferhave been attached together in a planar manner by the step of admittingair into the vacuum chamber, grinding down the product wafer to athickness below 100 micrometers.
 22. The method according to claim 21,which further comprises, after performing the step of grinding down theproduct wafer, etching down the product wafer to a thickness of aslittle as 15 micrometers.
 23. The method according to claim 21, whichfurther comprises, after performing the step of grinding down theproduct wafer, cutting the product wafer into individual chips.
 24. Themethod according to claim 20, which further comprises: providing thechuck as a heatable chuck; performing the attaching step by attachingthe adhesive film onto the carrier wafer; and after the carrier waferand the product wafer have been pressed onto each other in a planarmanner by the step of admitting air into the vacuum chamber, heating upthe chuck in order to heat up the carrier wafer and the product wafer toa melting temperature of the adhesive film.
 25. The method according toclaim 20, which further comprises: after the carrier wafer and theproduct wafer have been attached together in a planar manner by the stepof admitting air into the vacuum chamber, grinding down the productwafer; and subsequently adhesively attaching the product wafer and thecarrier wafer onto a saw frame covered with film.
 26. The methodaccording to claim 20, which further comprises separating the productwafer from the carrier wafer by heating the chuck above a releasetemperature of the film or the adhesive layer.
 27. The method accordingto claim 26, which further comprises sawing the product wafer intochips.